Structural Biochemistry/Anaerobic Respiration (Fermentation)

Definition
Anaerobic respiration is the formation of ATP without the presence of oxygen. This method uses the electron transport chain without the presence of oxygen as the electron acceptor. Although oxygen is highly oxidizing, it is only used during aerobic processes. In anaerobic repiration, less oxidizing molecules such as sulfate (SO42-), nitrate (NO3-), or sulfur (S) are used as electron acceptors. Thus, less energy is formed per molecule of glucose during anaerobic respiration. Most prokaryotes that live under environmental conditions that lack oxygen uses aerobic respiration, although humans too, use it sometimes as well (Lactic Acid Fermentation).

In this process, specifically with the absence of Oxygen during respiration process, organisms have evolved with mechanisms to recycle Nicotinamide Adenine Dinucleotide (NAD+) for glycolysis to continue in order to synthesize Adenosine Triphosphate (ATP) molecules, known as "energy currency" of cells. This process evolves into two different mechanisms, although both share the name of "fermentation":
 * 1) Ethanol Fermentation occurs in bacteria, yeast,...
 * 2) Lactic Acid (or Lactate) Fermentation occurs in animals (humans,...)

Ethanol Fermentation
After glycolysis, without Oxygen, pyruvates are carried into the ethanol fermentation cycle:
 * 1) With cofactors such as pyruvate decarboxylase enzyme, TPP (thiamine pyrophosphate), pyruvate is converted into acetaldehyde and carbon dioxide.
 * 2) Acetaldehyde, under oxidation-reduction reaction catalyzed by the enzyme ethanol dehydrogenase, is reduced to ethanol, with NADH being oxidized to NAD+.
 * 3) NAD+ is then used for glycolysis to synthesize ATP, and if Oxygen is still absent, fermentation continues. Therefore, ethanol and CO2 are waste products of this fermentation process.

Note that because of this reaction, when using yeast or some bacteria in food processing, there are a slight smell of alcohol (from ethanol) and gas coming off the mixture (from CO2)

Lactic Acid Fermentation
Similarly, in animals (humans,..), pyruvates must go into fermentation cycle if no oxygen is carried into the cells. However, there is one difference:
 * Under reduction-oxidation reaction catalyzed by the enzyme lactate dehydrogenase, pyruvate is reduced into lactate, with NADH being oxidized to NAD+.

Note that in animals, overworked muscles, whose cells cannot receive sufficient amount of oxygen to compensate for the loss of ATP, produce a lot of lactic acid as waste product of this fermentation process. Fermentation is diverse. There are two types of fermentation: homofermentative and heterofermentative. Homofermentative include lactic acid, ethanol, and carbon dioxide. However, heterofermentative include propionic acid, acetic acid, carbon dioxide, hydrogen gas, butyric acid, butanol, acetone, isopropyl alcohol, and succinic acid. Furthermore, in lactic fermentation there are homolactic versus heterolactic. The heterofermentative lactic acid bacteria lack aldolase and that the first steps of heterofermentative pathway are from the Pentose-Phosphate Pathway. An example of lactic fermentation is an organism called Lactobacillus acidophilus. This organism is a gram positive, it is homofermentative, it is belong to Firmicutes group, and it can be found in the normal human flora.

Fats and Protein
Sometimes, glucose may not be available for cellular respiration. Without this essential fuel, cellular respiration would stop completely and death would occur. So there must be backup storage that keep fuel just in case glucose levels are insufficient. When glucose is low, the body uses carbohydrates, such as glycogen stored in the liver and muscles. When carbohydrates deplete, fat is used next and, as last resort, protein is used. When fats and proteins are used, they must first be converted to glucose or some derivative. Fats are broken down to fatty acids and glycerol. Fatty acids can turn into acetyl-CoA by beta oxidation. Glycerol can turn into an intermediate of glycolysis. Proteins are broken down into amino acids. They are modified into -keto acids then converted into the various intermediates of Kreb's cycle.